Fluid coke calcining process employing a dual bed



March I3, 1956 w. J. METRAILER FLUID COKE CALCINING PROCESS EMPLOYING ADUAL BED Filed Jan. 25, 1955 FLUID COKE SUPPLY PREIIEATEO O CONTAININCCA5 APRC cons mo RATE CONTROL C PREIIEATED 0 4 commmc GAS AUXILI ARYFUEL COKE NITNORAIAL STRIPPINC CA8 RATE CONTROL CALCINED FLUID CONE TOSTORACE INVENTOR WILLIAM J. NETRAILER BY A 64 ATTORNEY WilliamJosephMetrailer, Baton Rouge, La., assignor to v United States PatentOEsso Research-and Engineering Company, a corpcra- This-inventionrelatesto improvements in calcining fluid coke. More particularly it relates toa dual bed operation wherein a fluidized bed of. the coke being treatedis superimposed on a moving bed.

There has. recently been developed an improved process known as thefluid coking process for the production of fluid coke and the thermalconversion of heavy hydrocarbon oils to lighter fractions, e. g., seeSerial No. 375,088, filed August 10, 1953. For completeness the processis described in further detail below although it should be understoodthat the fluid coking process itself is no part of this invention.

The fluid coking .unit consists basically of a reaction vessel or cokerand a heater or burner. vessel. In a typical operation the heavy oil tobe processed isinj'ected into the reaction vessel containing'a denseturbulent fluidized bed of hot inert solid particles, preferably cokeparticles. A transfer line or staged reactors can be employed.Uniform'temperature exists in the coking bed. Uniform mixing in the bedresults in virtually isothermal condi-' tions and effects instantaneousdistribution of the feed stock. In the reaction zone the feed stock ispartially vaporized and partially cracked. Product vapors are removedfrom the coking vessel and sent to a fractionator for the recovery ofgas and light distillates therefrom. Any heavy bottoms is usuallyreturned to the coking vessel. The coke produced in the process remainsin the bed coated on the solid particles. Stripping steam is, injectedinto the stripper to remove oil from the coke particles prior to thepassage of the coke to the burner.

The heat for carrying out the endothermic coking reaction is generatedin the burner vessel, usuallybut not necessarily separate. A stream ofcoke is thus transferred from the reactor to the burner vessel, such asa transfer line or fluid bed burner, employing a standpipe and risersystem; air being supplied to the riser for conveying the solids to theburner. Sufficient coke or added carbonaceous matter is burned in theburning vessel; to bring the' solids therein up to atemperaturesuflicient to maintain the system in heat balance. The burnersolids are maintained at a higher temperature than the solids in thereactor. About 5% of coke, based on the feed, is burned for thispurpose. This may amount to approximately 15% to 30% of the coke made inthe process. The net coke production, which represents the coke makeless the coke burned, is withdrawn.

Heavy hydrocarbon oil feeds suitable for the coking process includeheavy crudes, atmospheric and crude vacuum bottoms, pitch, asphalt,other heavy hydrocarbon petroleum residua or mixtures thereof.Typically, such feeds can have an initial boiling point of about 700 F.or higher, an A. P. I. gravity of about to 20, and. a Conradson carbonresidue content of about to 40 wt. percent. (As to Conradson carbonresidue see A. S. T. M. Test D--18052.)

, The method of fluid solids circulation described above is well knownin the prior art. Solids handling tech 2,738,316 Patented 'lVlar. 13 1956 2. nique is described broadly in. Packie Patent 2,589,124, issuedMarch 11, 1952.

The fluid coke. product particles have a particle diameterpredominantly, i. e.,, about 60 to 90 wt. percent, in the range of 20 tomesh, a sulfur content in, many cases; above 6 wt. percent, and avolatile content of 2 to 10 wt. percent. They have a real density ofabout 1.4 to 1.7 which is too low for use in the manufacture of carvbonelectrodes in making aluminum and other purposes. Increased density andlower sulfur and. volatile content are particularly necessary before thefluid coke is suitable for manufacture into electrodes, one of the majoruses of petroleum coke. These can be accomplished. by calcining the cokeat high temperatures, e. g., minimum temperatures of 2100 F. or higher.These temperatures and the times required make the calcining operationrelatively difiicult and expensive.

In the calcination a considerable quantity of volatile matter is givenoff from the coke. This volatile matter, which comprises principallyhydrogen and methane, together with heavy hydrocarbons, can cause thefinely dividedfluid coke to agglomerate and bridge in a calcining vesselwhen the calcining is done while the fluid coke is, in the form of amoving bed. This obstructssolids flow and interferes with smoothoperation of the calcining unit.

This invention provides an improved process for calcining the fluid cokewhich overcomes these difliculties.

.The process. comprises calcining the coke. at a temperature in therange of 2000 to 3000 F. and preferably 21300 to 2700- F. utilizing adual bed. The. fluid coke .is maintained in. theform of a denseturbulent. fluidized bed directly superimposed on a moving bed of thecoke.

The holding time in the-fluid bed isv in the range of 0.1 to 0.01 of thetotal treating time whereby substantially all of the volatile matter isremoved from the coke while it is in the form of the fluidized bed.Additional fluid coke charge stock is fed to the fluid bed and calcined"product coke is withdrawnfrom the moving bed.

The total holding time consequently is in the range of 0.2 to 20- hours,and preferably 0.3 to- 12 hours with the distribution as indicatedabove.

The terms dense turbulent fluidized bed and moving bed are employed intheir usual engineering connotations, i. e., see IndustrialandEngineering Chemistry, volume 41., page 1249 (June 1949).

This invention will be better understood by reference to the followingexample, description and the flow dia gram shown in the drawing.

Hot product coke from. the fluid coking process is fed from supply line1, through a flow control valve 2, to. line 3 and/or 3A. The coke fedthrough line 3 is picked'up by a preheated; gas containing. oxygenentering'through line 4 and the mixture is. distributed uniformly atthebottom of thefluidized bed 6 in fluidized bed section 9. Coke fedthrough line 3A is discharged. uniformly. into the: dispersed phaseabove the fluid bed which hasan upper level 7. If an auxiliary fuel isdesired it would be fed through line 5, mixed with the 02 containing gasand distributed uniformly at the bottom of the fluid bed 6. The rate ofcoke addition is controlled by a slide valve actuated by a pressuredifferential controller 8 set to maintain a constant level in the fluidbed section. Coke is withdrawn from the calcining vessel through acooler 15 with the rate of withdrawal being controlled by a slide valve16. The fluid bed 6 is in direct communication with the moving bed 11 inmoving bed section 10, and as coke is withdrawn from the bottom of themoving bed section, additional coke will enter the top of this section,falling by gravity from the fluid bed section. The total holding time isregulated by controlling the rate of coke withdrawal through slide valve16. The relative holding time in the fluid bed and moving bed sectionsis regulated by coke level in the fluid bed section.

A stripping gas such as hydrogen, natural gas or nitrogen is introducedinto the bottom of the moving bed and/or cooler through lines 13, 14and/or 14A, to remove any volatile materials formed in the moving bedsection. Provisions are also made to introduce a preheated oxygencontaining gas through line 19 and an auxiliary hydrocarbon fuel line 18through line 14 to the bottom of the moving bed section to provide heatonly when starting up the unit. In normal operations all heat issupplied in the fluidized bed section. Evolved vapors are taken offthrough cyclone 17 and line 12 along with other gases. The superficialgas velocity in the fluid bed section is between 0.5 and 8 feet persecond and preferably 2 to 5 feet per second. The superficial gasvelocity in the moving bed section is between 0.005 and 0.05 feet persecond. The cross section area of the fluid bed section 9 can be reducedto as much as one twentieth that of the moving bed section to minimimizethe quantity of fluidizing gas necessary.

As a specific example of the above described process fluid coke at 1200F. from the fluid coking process is fed to a calcining vessel at a rateof about 60 tons per day. The calcining vessel proper is about 11 feetin internal diameter and 30 feet tall, not including the attendantcooling section and fine solids recovery equipment. The moving bedsection is 20 feet deep and the fluidized bed section about 2 feet deep.This provides a total holding time of about 1 hour in the calciningvessel with slightly less than per cent of the total holding time beingin the fluid bed section because of the lower densities in the fluid bedsection. Air, preheated to about 1200" F. by indirect heat exchange withthe exit gases from the calcining vessel, is distributed through porousrefractory tubes uniformly across the bottom of the fluidized bed. Therate of air addition is adjusted to maintain a temperature of 2700" F.in the fluidized bed. This rate should be approximately 500,000 S. C.F./hr. for this size plant but varies somewhat with the burningcharacteristics of the coke. This gives a superficial velocity of about5.0 feet per second in the fluidized bed section. The fluid coke entersthe calcining vessel through the top of the vessel and is distributeduniformly in the dispersed phase above the fluidized bed. Nitrogen isadded at the bottom of the moving bed at a rate of about 2000 S. C.F./hr. to maintain a superficial velocity of about 0.02 foot per secondin the moving bed section. The calcining vessel is thermally insulatedor otherwise protected from excessive heat loss so that the temperatureat the bottom of the moving bed is about 2600 F. The treated cokewithdrawn through line is cooled to about 500 F. by indirect heatexchange and withdrawn through a slide valve.

The calciner itself advantageously can be constructed of two materials,i. e., an inner layer of a shock resistant porous refractory such ascarbon, graphite or silicon carbide. This can be coated with arelatively thin layer of a non-porous refractory which has only poorresistance to thermal shock, e. g., porcelain, vitrified alumina andstabilized zirconia.

Some of the advantages of this invention are as follows:

Agglomeration and bridging are avoided.

The removal of the volatile matter early in the process means thatsmaller equipment can be used subsequently, i. e., the moving bedsection of the calciner, the solids recovery system and piping.

The evolved volatile matter form a part of the fluidizing gas.

Oxygen containing gases are injected into the small fluidizing sectionso only this section has to be constructed with expensive oxygenresistant refractories.

Advantage can be taken of the high heat transfer to a fluid bed and atleast part of the heat can be supplied by indirect heat transfer in thefluid bed.

The sulfur content of the coke is reduced to below 3 wt. per cent andeven below 2 wt. per cent by the process of this invention and the realdensity raised to a minimum of 1.8.

The conditions usually encountered in a fluid coker for fuels are alsolisted below to further illustrate how the coke was prepared.

Conditions in fluid coke! reactor It is to be understood that thisinvention is not limited to the specific examples which have beenoffered merely as illustrations and that modification may be madewithout departing from the spirit of the invention.

What is claimed is:

1. In a process for dcsulfurizing and increasing the density of fluidcoke particles by calcination at a temperature in the range of 2000 F.to 3000 F. the improvement which comprises the steps of maintaining thefluid coke in the form of a dense, turbulent, fluidized bed directlysuperimposed on a moving bed of fluid coke, the holding time of thefluid bed being in the range of 0.1 to 0.01 of the total treating timewhereby substantially all of the volatile matter on the coke is removedwhile the coke is in the form of the dense, turbulent, fluidized bed;feeding additional fluid coke charge stock to the dense, turbulent,fluidized bed and withdrawing calcined product coke from the moving bed.

2. The process of claim 1 in which the total treating time is in therange of 0.2 to 20 hours.

3. The process of claim 2 in which the coke particles being calcined arebrought to the required temperature by utilization of an oxygencontaining gas as the fluidizing gas for the dense, turbulent, fluidizedbed.

No references cited.

1. IN A PROCESS FOR DESULFURIZING AND INCREASING THE DENSITY OF FLUIDCOKE PARTICLES BY CALCINATION AT A TEMPERATURE IN THE RANGE OF 2000* F.TO 3000* F. THE IMPROVEMENT WHICH COMPRISES THE STEPS OF MAINTAINING THEFLUID COKE IN THE FORM OF A DENSE, TURBULENT, FLUIDIZED BED DIRECTLYSUPERIMPOSED ON A MOVING BED OF FLUID COKE, THE HOLDING TIME OF THEFLUID BED BEING IN THE RANGE OF 0.1 TO 0.01 OF THE TOTAL TREATING TIMEWHEREBY SUBSTANTIALLY ALL OF THE VOLATILE MATTER ON THE COKE IS REMOVEDWHILE THE COKE IS IN THE FORM OF THE DENSE, TURBULENT, FLUIDIZED BED;FEEDING ADDITIONAL FLUID COKE CHARGE STOCK TO THE DENSE, TURBULENT,FLUIDIZED BED AND WITHDRAWING CALCINED PRODUCT COKE FROM THE MOVING BED.